The present invention relates to pumps, and more particularly to pump apparatus and methods for pumping molten metal.
The use of pumps to pump molten metal such as aluminum or zinc is known in the art. Generally, molten metal pumps comprise centrifugal pumps modified to provide processing of the molten metal. To that end, circulation pumps are used to equalize temperature and improve homogeneity of mixture in a molten metal bath, transfer pumps are used to convey or transfer molten metal between locations and gas-injection pumps are used to circulate and inject gas into a molten metal to modify its composition as by removing dissolved gases or dissolved contaminant metals therefrom.
The pumps typically include a base or casing having a pumping chamber and an impeller received within the chamber. The base includes inlet and outlet passages for intake and discharge of the molten metal being pumped. The pump may be a volute pump wherein the pumping chamber has a volute shape comprising a spiral configuration of circumferentially increasing cross sectional area approaching the pump outlet passage. It is also possible to provide the pump with a pumping chamber having a generally circular shape.
The pump base together with the impeller are submerged in the molten metal and connected via a plurality of support posts to a drive arrangement positioned above the level of the molten metal. The impeller is supported for rotation within the pumping chamber by a rotatable shaft coupled to the drive arrangement. In typical installations, the drive shaft may be of various lengths, e.g. one to four feet in length or longer, in order to provide adequate clearance above the molten metal level.
A typical impeller includes at least two axially extending vanes and a radially extending member which forms a base when located below the vanes. In this manner the impeller provides a vane array with adjacent vanes cooperating with the base to form vane pockets. During pumping, molten metal is axially introduced into the pockets and laterally ejected due to centrifugal force.
The necessary spacing between the driver and impeller results in the use of an elongate drive shaft fixed to the impeller. This requires a relatively high degree of balance during operation and adequate bearing support between the impeller/shaft assembly and the housing. Operating vibration may damage the pump and/or limit its pumping efficiency.
The impeller may be fractured or otherwise damaged due to the vibrations and failure to maintain operating clearances. In molten metal pumping systems, bearings may be considered to operate on films of molten metal and poor concentricity yields reduced clearances which may cause the films to break down or not form so as to give rise to refractory material wear of increased rate.